If you get this error trying to build your Silverlight application in TeamBuild: C:\Program Files (x86)\MSBuild\Microsoft\Silverlight\v4.0\Microsoft.Silverlight.Common.targets (104): The Silverlight 4 SDK is not installed, and you know that you have it installed on your build machine, don’t panic. There is an easy solution to this:
Go to your build definition, select the “Process” tab, and under “Advanced” change the value for MSBuild platform from “Auto” to X86
This happens when you are running your builds on a 64-bit build server. The reason for this is that the Silverlight SDK seems to have trouble compiling under a 64-bit environment, and your 64-bit build server will try to use those settings when you set it to “Auto”.
This will turn your build from 
to 